Building block



FIG. 1 shows a top plan view of a building block.

FIG. 2 shows a front elevation view thereof, the rear elevation and side elevation views being identical.

FIG. 3 shows a bottom plan view thereof.

FIG. 4 shows a top and side perspective view thereof, the reverse top and side view being identical; and,

FIG. 5 shows a bottom and side perspective view thereof, the reverse bottom and side view being identical. 

CLAIM The ornamental design for a building block, as shown and described. 